The grafting of polyolefins consisting largely of polypropylene with polar chemical modifiers, such as maleic anhydride (MAH) and acrylic acid is known. The grafting reaction can be performed in solution, in gas phase, or by surface modification. However, the most common grafting process is performed during melt processing, for example in single or multiple screw extruders, rubber masticators, Banbury mixers, Brabender mixers, roll-mills and the like. Such procedures are well-known and described in the technical literature. The grafting of polypropylene with unsaturated monomers including maleic anhydride is described, for example, in S. B Brown, Reactive Extrusion, Chapter 4, published by Polymer Processing Institute (1992). U.S. Pat. No. 5,439,974 addresses adhesive blends suitable for adhesion to one or more polypropylene substrates consisting essentially of a mixture of an impact copolymer and a graft reaction product of a propylene polymer and a carboxylic acid or a derivative thereof. The polymeric backbone, which is grafted, is an impact copolymer, defined as a mixture of polypropylene and ethylene/propylene copolymer.
WO02/36651, the disclosure of which is hereby incorporated herein by reference, describes methods for grafting propylenic elastomers, also referred to herein as propylene-ethylene copolymers. WO02/36651 discloses grafting of propylene ethylene copolymers having a low ethylene content (between 8 and 32 mole %), and with predominantly isotactic polypropylene segments (>80% mm triad by 13C NMR), giving polypropylene type crystallinity, with MAH. These polymers, comprising low levels of ethylene content, permit higher levels of grafting.
Polyolefins, which have polar groups due to such grafting reactions, are used in blends with engineering thermoplastics. These may have a polyamide (PA) matrix and a dispersed phase formed by the grafted polyolefin, which acts as the impact modifier (IM) so as to reduce brittle failure. The IM may also include a non-grafted EP rubber (EP) or a low-density amorphous or semi-crystalline polyolefin (VLDPE). Certain publications suggest the use of an ungrafted propylenic elastomer as impact modifier. See, for example, WO99/65982 and WO2000/01766. In such a case, the grafted polyolefin acts to compatibilize the PA and the ungrafted IM. A three-component blend of PA, ungrafted polyolefin and MAH-grafted ethylene propylene rubber results.
Impact modification is also used where the matrix polymer is polypropylene having a high degree of crystallinity as generally indicted by melting point of over 100° C. The polypropylene matrix polymer has different properties from the polyamide matrix polymer, which is reflected in the heat resistance and strength and reduced moisture sensitivity. Blends of polypropylene and polyamide have hence been employed with an additional impact modifier to obtain a desirable overall balance of cost and performance. Stiffness can be improved by incorporating the polyamide as a dispersed phase in a polypropylene matrix continuous phase. The cost of the composition can be reduced at marginal loss of performance by incorporating polypropylene as a dispersed phase in a PA matrix continuous phase.
The manner in which the different phases are intermingled has influence on the overall cost performance balance. MAH grafted polypropylene has been used to improve performance of polypropylene-polyamide alloys. EP-658640 discloses a three-component blend of polypropylene, PA and MAH-grafted polypropylene.
WO02/36651, beginning on page 12 line 8, discusses blends of the grafted propylene based elastomers with other alpha-olefin polymers and copolymers, e.g., polypropylene for fabrication into objects used in a variety of applications. Beginning on page 26 line 1, WO02/36651 discusses use as compatibilizers of polyamide-polypropylene blends to increase the percent of elongation at break point. A composition comprising polypropylene, a polyamide, and a MAH-grafted propylene based elastomer would result.
U.S. Pat. No. 4,795,782 discloses three component blends of PA, MAH-grafted polypropylene and MAH-grafted styrene-butadiene-styrene tri-block copolymers. Polymer Vol. 36 pages 4587-4603, 1995 discloses three-component blends of PA, polypropylene and MAH-grafted EP rubber. Run 12 in Table 1 of GB2226035 discloses blends of PA, polypropylene, EP copolymer and MAH-grafted propylene based Impact Copolymer. JP-03252436 discloses the use of compositions of polypropylene, PA, EP copolymer and MAH-grafted polypropylene.
Embodiments of the present invention may have one or more advantages over previously known materials, such as providing compositions containing both PA and polypropylene with improved cost performance balance and improved balance of impact strength and stiffness. In some embodiments, the impact strength may be significantly improved without substantive reduction of stiffness, particularly for compositions having a relatively low stiffness.